This invention relates generally to an apparatus and method for improving traction efficiency of agricultural wheeled tractors and reference is made to U.S. Pat. No. 4,402,357 the disclosure of which is incorporated herein by reference. The invention disclosed in this patent includes a method by which the traction intensifying means are engaged and retracted for obtaining added utility.
In the above referenced patent, shortcomings of the pneumatic agricultural tires are described and analyzed. Some further observations are as follows.
To efficiently convert horsepower into usuable drawbar pull is a primary object of agricultural drawbar tractors. In this respect, the weakest, most inefficient link in that chain is the tire itself, or more specifically, the tire-soil interface. In Handbook of Agricultural Tyre Performance (Dwyer, et al, 1976, National Institute of Agricultural Engineering, Silsoe, England) test results are shown for tires ranging in size from 12.4-36 through 18.4-38 operating on five types of fields. Per these tests, the maximum "terra-dynamic" efficiency averages 65%; varying from 55% on wet, loose earth to 75% on dry grassland. By contrast, mechanical efficiency of a tractors powertrain is typically about 94%. Thus, if its engine delivers 106 horsepower to the flywheel, 100 hp is available to its drive wheels; but, on the average, only 65 hp are available at the drawbar for the implement; and a modest 55 hp if the ground consists of wet, loose earth. Further shown are the drawbar pulls obtained in the same tests and given at 20% rate of tire slippage and at tire loads, which include a 20% add-on weight, as permitted by manufacturers at travel speeds below 20 km per hour (12 miles per hour). Per these tests, the drawbar pull averages 50% of tire load.
The deviation in tire efficiency of plus and minus ten percentage points reflects the multitude of soil conditions encountered. This necessitates compromise in tire design; it is noteworthy that a leading tire manufacturer recommends no less than seven types of tires to cover five applications of usage.
While tire manufacturers are offering a great number of tire configurations, the tractor design has undergone a series of evolutions in the last half a century; now seemingly entering its fourth phase for optimum drawbar performance
first: adding ballast to two-wheel drives (still prevails); PA1 second: making bigger two-wheel drives; PA1 third: making large, articulated type four-wheel drives; PA1 fourth: furnishing so called Factory Installed Front Power Assist Option; PA1 at 15% slippage: steel wheels/tires=0.86/0.22; (3.9); PA1 at 20% slippage: steel wheels/tires=0.90/0.29; (3.1); PA1 at 30% slippage: steel wheels/tires=0.95/0.36; (2.6);
The latter achieves improved traction by diverting power, originally intended for driving rear wheels only, also to the steering wheels. This option is not only expensive, adding some 20% to tractor price, but the improvement in drawbar pull is nominal; averaging 17% according to the english book Agricultural Tyres (Inns & Kilgour, 1978, Dunlop Ltd., London, England). Nevertheless, its demand in the U.S. is said to be growing at a 30% annual rate. (Truck & Off-Highway Magazine, June/1983).
Early tractors achieved their high traction efficiency from utilizing the higher soil values of sub-surface layer, as elaborated upon in the above referenced patent. The superiority of this approach to obtain drawbar pull is quantified from comparative tests, published in a swedish paper, entitled Tendencies in Tractor Development, Bigger Tractors--Higher Drawbar Pull--Four-Wheel-Drives (Nordstrom, 1966, Statens Maskinprovningar, Ultuna, Sweden). Here, the coefficient of drawbar pull in sugar-beet fields of different conditions were measured for various drive wheel combinations and arrangements. In one series of tests, steel wheels of early tractors were compared with tires, both tested on same two-wheel drive tractor. Per these tests, the coeff. of DBP (coefficient of drawbar pull)
Above tests were made on wet beet field and, in regard to the tests performed at 20% slippage, the paper states, in part: " . . . The coeff. of drawbar pull, and thus the drawbar pull at same tractor weight, is--at 20% slippage--over three times as high with the steel wheels. At change-over to steel wheels on the same tractor, its weight becomes, however, considerably less if no ballast is added to the steel wheels. With the tractor used in the tests, the weight with steel wheels was only about 70% of the weight with tires. Nevertheless, the drawbar pull was more than twice as high with steel wheels." To assess the significance of the difference in coeff. of DBP in above test series additional data are required; these are given in Table I below. To compute the corresponding differences in productivity and hourly cost savings, say that: the load from the implement is such so as to require 20% slippage when tractor is equipped with tires; further, in both configurations, the tractor is operating so as the speedometer reads five miles per hour.
TABLE I ______________________________________ Two-Wheel Drive Tractor operating on wet sugar- beet field, and equipped with Tires Steel Wheels Size: 14-34 Type: Skeleton ______________________________________ Coeff. of DBP at slippage rate 30% 0.36 0.95 25% 0.33 0.93 20% 0.29 0.90 15% 0.22 0.86 10% 0.14 0.80 5% N.G.* 0.67 21/2% N.G. 0.40 2% N.G. 0.25 Weight w/o driver, Tot. Tractor, kg 4420 2900 On Drive Wheels, kg 3170 1870 % of Total 75% 65% ______________________________________ *N.G. = Not Given in test report
Analysis: When tractor is equipped with tires and slippage rate is 20% it means the prevailing coeff. of DBP is 0.29; and, as the weight on drive tires is 3170 kg, the drawbar pull exerted by the tractor is 0.29.times.3170=919 kgf. At a speedometer reading of 5 miles per hour and the slippage rate is 20%, the actual travel speed is 5.times.(1-0.20)=4,00 mph. To exert same drawbar pull of 919 kgf when tractor is equipped with steel wheels, its coeff. of DBP must be 919/1870=0.49. Extrapolating from Table above, the required slippage rate is 3%. At a speedometer reading of 5 miles per hour and the slippage rate is 3%, the actual travel speed is 5.times.(1-0.03)=4.85 mph. Thus, by change-over to steel wheels, the travel speed has been increased by 21% (4.85/4.00-1), even though the tractor's weight was reduced from 4220 kg to 2900 kg, or by 31% (4220-2900=1320; 1320/4222=0.31). Say, further, that hourly operating cost is 30 dollars; at five "speedometermiles" per hour, the cost per mile is 6 dollars. For each ten hours of operation, the tractor moves 40 miles when equipped with tires while this distance becomes 48.5 miles if equipped with steel wheels. Thus, not only is the productivity of the tractor here improved by 21% but 51 dollars (8,5.times.6=51) is saved in operating costs for every ten-hour shift. And this, because the steel wheels are capable of utilizing the ground more efficiently, even though their weight upon it is 1300 kg less than with tires, (3170-1870=1300).
The above set of circumstances are well known in the art, but reviewed here to illuminate the high potential for productivity increase, energy and cost savings, which the concept of steel wheels offer. On the other hand, the tire offers an all together overriding advantage of on-pavement travel. From this, it becomes apparent that a combination of both concepts will provide a useful solution. The present invention embodies such an arrangement and employs a simple mechanism, by which the advantages of tires as well as of steel wheels are utilized and offering an expeditous conversion from one concept to another and vice versa.